1. Technical Field
The present invention relates to a flow control device and particularly, but not exclusively, to a flow control device for controlling fluid flow into a turbine.
2. Description of the Related Art
Steam turbines include admission control devices to control stream flow and turbine speed. Without the restriction imposed by an admission control device the steam flow into the turbine would be determined by the physical dimensions of the turbine inlet stage (the “swallowing capacity”) and by the steam conditions at the inlet.
Generally, the speed and/or power of a steam turbine is controlled by one of two governing modes, determined by the initial design of the turbine.
In a throttle governing mode (generally used for power generating steam turbines and for all large steam turbines), steam is supplied uniformly to the whole portion of the steam turbine. Steam control valves are simultaneously opened and closed at the same rate to control the flow of steam to the turbine. A throttle governing mode is characterised by the use of either a single control valve or multiple valves arranged in parallel and operating in unison.
Throttle governed control restricts the flow to the first stage by a control valve, reducing the pressure at the first stage so that steam flow through the turbine is reduced. The pressure reduction through the partially open control valves increases the entropy of the steam and leads directly to a loss in the work available from the fluid.
The alternative governing mode (nozzle governing mode) is used in industrial and smaller steam turbines, typically rated at less than 100 MW. In this case the first stage nozzles are arranged in separate groups, each independently supplied with steam via a control valve.
Nozzle governed control operates by successively opening the control valve to each nozzle (or nozzle group) such that each nozzle (or group) is individually sequentially opened. As soon as the capacity of a nozzle (group) is reached the next nozzle (group) in order is progressively brought into operation. As a result at any point only one group is throttled while all other groups in operation are functioning at their design point. This mode delivers enhanced part load efficiency compared with throttle governed control.
A comparison of the operating characteristics of turbines operating under the throttle governed mode and nozzle governed mode is shown in FIG. 1 where the upper chain dotted line shows the relationship between power output and steam flow for a turbine operated under a throttle governed mode and the lower dotted line shows the same relationship for an ideal turbine operated under a nozzle governed mode of operation, i.e. with an infinite number of independently controlled nozzles.
The central solid line shows the performance of a real turbine operated under a nozzle governed mode of operation with three independent arcs. This characteristic meets the lower line at each nozzle point, i.e. a point at which the valves are either fully open or fully closed with no valve in a throttling (partially open) condition.
One of the key disadvantages of the nozzle governing mode is that the steam velocity incident on rotating blades of a turbine rotor varies during each turn of the rotor. This cyclic loading subjects the blades to fatigue stresses, necessitating the use of a more robust blade design. Since the design of the rotating blades is affected by the use of the nozzle governing mode, it is generally not feasible to change a steam turbine from a throttle governing mode to a nozzle governing mode without major modifications to the rotor blades.
Larger utility steam turbines use the throttle governing mode as this is the technically feasible and economic design. However, there are a number of circumstances where this design results in throttling losses in normal operation:
(i) where the turbine is dispatched below full output at, for example, 90% output, to provide a margin in case other generating units breakdown, resulting in a shortfall in generation to meet demand. In this case the running turbines are called upon to increase output by opening the throttles; and
(ii) where the steam supply system has an output that is limited by temporary or seasonal derating or degradation. Such conditions apply to steam production from gas turbine heat recovery steam generators and from some types of nuclear steam cycle.
In both these cases the reduced output condition becomes a common running condition, with the associated loss of efficiency leading to either increased fuel consumption or reduced output affecting significant numbers of operating hours of the generating unit.